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Servo Transperency

AS50, Dalamot Norway, 2011 (LOC HF FIRE)

On 4 July 2011, an Airlift Eurocopter AS 350 making a passenger charter flight to a mountain cabin in day VMC appeared to suddenly depart controlled flight whilst making a tight right turn during positioning to land at the destination landing site and impacted terrain soon afterwards. The helicopter was destroyed by the impact and ensuing fire and all five occupants were fatally injured. The subsequent investigation came to the conclusion that the apparently abrupt manoeuvring may have led to an encounter with ‘servo transparency’ at a height from which the pilot was unable to recover before impact occurred.

Description

On 4 July 2011, a Eurocopter AS 350B3 Ecureuil being operated by Airlift on a passenger charter flight to transfer four people to a cabin site in the mountains in day VMC appeared to suddenly depart controlled flight whilst positioning at a low height above terrain when preparing to land at the destination landing site and impacted terrain soon afterwards. The helicopter was destroyed by this impact and ensuing fire and all five occupants were fatally injured.

Investigation

An Investigation was carried out by the AIBN. No recorded flight data was available to assist the Investigation because the aircraft involved had not carried, nor been required to carry, an FDR. The Non Volatile Memory (NVM) of the GPS receiver on board the helicopter was recovered and successfully accessed and was of some value in confirming the operation of the helicopter and corroborating eyewitness accounts of the final minutes of the flight. The NVM from the Digital Engine Control Unit (DECU) and the Vehicle and Engine Multifunction Display (VEMD) was also recovered but both had been rendered unreadable by fire damage.

The weather conditions at the accident site at the time of the crash had been good with light winds and a surface temperature in the vicinity recorded as 23º C. The altitude of the site was in excess of 3500 feet amsl.

Without recorded flight data, it was impossible to know with certainty what flight attitudes and loads the helicopter was subjected to, what airspeed it held and the exact flight path it followed in the turn preceding the crash. However, the Investigation was able to calculate the approximate forward speed at ground impact as 105 knots based on tracks at the accident site. It could also be deduced that the helicopter had hit the ground at a nearly flat pitch angle and with a bank angle of about 45° to the right.

It was established that as the helicopter had started a descent to approach the intended landing site, it had been seen by witnesses to make a steep turn to the right. During that turn, control of the helicopter appeared to be lost with a continued high bank angle and steep descent being evident. Near to the eventual impact site, it had seemed to these witnesses as if control was about to be regained, but the helicopter had then impacted the ground hard about 500 metres short of the landing site and had immediately caught fire.

It was noted that the extensive damage to the helicopter meant that meaningful examination of components was almost impossible. However, it was possible to establish that the helicopter had been structurally intact prior to ground impact with the engine producing power. No evidence was found of any significant airworthiness defects or of any irregularities in maintenance which could have contributed to the accident. It was assumed that the attached external cargo basket would have had produced only a small reduction in the overall performance of the helicopter.

On the basis of descriptions of the observed flight path, the phenomenon of ‘servo transparency’ or ‘jack stall’ was identified as a possible cause of the sudden loss of control that had evidently occurred. It was noted that under certain conditions, usually abrupt manoeuvring, it is possible to encounter the most marked version of it during a right turn as being made by the accident helicopter. It results from the hydraulic system reaching the limit of its capability. Whilst recovery from an encounter with the phenomenon is always possible given sufficient height above terrain, if the condition occurs in sufficiently close proximity to terrain or to obstacles, it would present a very hazardous prospect.

Occurrence of this phenomenon, which is nevertheless a natural state for any helicopter with effects which are intended to be contained by a combination of design and compliance with the type operating limitations, has been particularly associated with high mass, high speed, high torque and high density altitude. It was noted that the latter means that it will occur more easily when flying a heavily loaded helicopter in the mountains on a hot day, as was the case in the investigated accident.

It was not considered that “any misjudgement (by the accident pilot) can be attributed to lack of knowledge or skill” but it was considered that there appeared to be “insufficient general knowledge in the helicopter community of the dangers associated with servo transparency”.

The Investigation found that opinions on whether the servo transparency phenomenon should be or can be demonstrated / practiced in flight varied. Most Norwegian Operators of the accident type were found to provide theoretical classroom training but no in-flight practical demonstration. One of the reasons for this appeared to be the view that such demonstrations would involve operating outside of AFM limitations. It was noted, however, that in 2010, Eurocopter had issued training guidelines based on demonstration using the much less marked left bank case and that the accident pilot had been given practical training/demonstration.

Although the Operator involved had not been the subject of regulatory concern, evidence that hazardous manoeuvring with passengers on board may previously have occurred in their operations was found and the Investigation came to the conclusion that “the same applies for most of the operators in this part of the industry”.

In respect of the absence of a requirement for helicopters such as the one involved in the investigated event to carry a FDR, it was noted that such recorders not only provide a valuable data source to help establish what has happened in an air accident or incident, they can also facilitate improved safety in normal operations. It was considered that the difficulties encountered during this Investigation indicated “that it is time for the aviation authorities to require suitable recorders for lighter aircraft…..including light helicopters”.

The overall Conclusion of the Investigation was that it was “likely that abrupt manoeuvring initiated a sequence where control of the helicopter was partly lost for a period, and that the height was insufficient for the commander to recover in time”.

Safety Action taken by the aircraft operator Airlift following the accident was noted to have the introduction of limits of 30º bank and 15º pitch when at heights below 500 feet agl with passengers on board.

Three Safety Recommendations were made as a result of the Investigation as follows:

that the (Norwegian) Civil Aviation Authority, through its role as chair of the Committee for Helicopter Safety – Inland Operations, ensures that an industry standard for manoeuvring limitations is established for passenger transport. [SL 2012/08T]

that EASA requires the type certificate holder Eurocopter to issue a warning of the particular hazard when encountering servo transparency in a right turn, preferably as a permanent note in the Flight Manual of the helicopter models in question. [SL 2012/09T]

that EASA considers introducing requirements for flight recorders on more aircraft than those covered by the current regulations. [SL 2012/10T]